1. Field of the Invention
The present invention relates to a structure of an active matrix type reflection type liquid crystal panel.
2. Description of the Related Art
FIG. 8 schematically shows a section of a conventional active matrix type reflection type liquid crystal panel. In the structure shown in FIG. 8, a TFT (thin film transistor) constituted by a source region 708, a gate electrode portion 709, and a drain region 710 is disposed, and further a reflecting pixel electrode 706 is connected to the drain region 710 of the TFT.
Generally, in such a structure, the flatness of upper surfaces of the source electrode portion 708, the gate electrode portion 709, and the drain electrode portion 710, as well as the flatness between the pixel region 711 and other region is not considered much as shown in the drawing.
The disturbance of the flatness (that is, roughness) generally becomes about 300 nm to 700 nm.
The existence of such roughness causes disturbance 707 of liquid crystal molecules. However, in the case where the thickness of a liquid crystal layer is as thick as 7 .mu.m or 8 .mu.m or more, the roughness does not greatly influence the display.
That is, the influence of the disturbance of the liquid crystal molecules is not applied to the entire of the liquid crystal layer in the thickness direction, and the disturbance does not have much influence on the display.
However, in recent years, according to the pursuance of a high picture quality and the development of a liquid crystal material accompanying the pursuance, it is required to narrow the thickness of a liquid crystal layer further.
Especially, in a reflection type liquid crystal panel, since light passes through a liquid crystal layer twice, it is required to make the thickness 1/2 times that of a transmission type liquid crystal panel (although the actual situation is not so simple, the outline is true.)
Until now, the reflection type liquid crystal panel has not been required to have fine display characteristics and high speed moving picture display. Thus, it has not been necessary to compulsorily thin the liquid crystal layer to improve the display characteristics.
According to the knowledge of the present inventors et al, it is found that the reflection type liquid crystal panel is suitable for use in a projection type projector.
This is because the projector is required to have a compact picture size (if the picture size is large, the optical system becomes expensive), and with respect to a small picture size such as 2.5 inches or less in diagonal, the aperture ratio of the reflection type liquid crystal panel can be made higher than that of the transmission type.
In general, when the size of the picture becomes small, the rate of regions through which light does not pass, such as a TFT, a wiring line, and a capacitance electrode, becomes large in the transmission type, and the transmission loss in a transmission portion becomes also tangible. (According to the calculation by the present inventors, this tendency becomes tangible when the size of the picture becomes 2.5 inches or less.)
On the other hand, in the reflection type, it is possible to dispose a TFT, a wiring line, and a capacitance electrode at the lower portion of a reflecting electrode, and the reflection loss of the reflecting electrode can be made far smaller than the transmission loss of the transmission type.
A projector is required to have a performance to make high minute display. Thus, a reflection type liquid crystal panel used for the projector is required to have high display characteristics. Especially in the case of the projector, since a picture image is magnified several tens to 100 or more times, this request becomes severe.
By such reasons, high display characteristics are required also for the reflection type liquid crystal panel, and the thickness of a liquid crystal layer is required to be made thin to realize the request.
According to the knowledge of the present inventors, in order to obtain the required display characteristics, in the case of the reflection type liquid crystal panel, it is required to make the thickness of the liquid crystal layer about 2 to 4 .mu.m. This is required from the condition under which a contrast becomes highest.
In this case, if a level difference of the roughness at the surface in contact with the liquid crystal layer is 10% or more of the liquid crystal layer, the disturbance of liquid crystal orientation becomes tangible, and the deterioration of picture quality (especially the lowering of contrast) becomes remarkable.
The thickness "d" of the liquid crystal layer is determined by the condition under which the contrast becomes highest. The condition for maximizing the contrast is determined by the product (.DELTA.nd) of An (retardation) determined by the liquid crystal material and the thickness "d" of the liquid crystal layer. However, this value has a wavelength dependency, and has trouble properties that the dependency is different among liquid crystal materials, so that the optimization is not simple.
FIG. 7 shows the result of simulation assuming the reflection type liquid crystal panel. Here, the horizontal axis indicates the wavelength of incident light, and the vertical axis indicates the ratio of the incident light to output light (the ratio is defined as a transmissivity).
The plotting dots in FIG. 7 show various cases where the thicknesses of liquid crystal layers are changed.
The wavelength sensibility of a human eye is within the range of about 450 to 680 nm, and has a maximum value in the vicinity of 550 nm.
Thus, in the case where color display is made, it is important that the transmissivity within the range of 450 to 680 nm is as flat as possible in the curves as shown in FIG. 7.
Especially, the flatness within the range of 500 to 600 nm in which the wavelength sensibility is high, becomes important.
Of course, it is important that the transmissivity is as high as possible (that is, the transmission loss is as low as possible).
When this point is considered, in the result of simulation shown in FIG. 7, the cases in which the thickness of the liquid crystal layer is 2.86 .mu.m and 3 .mu.m, become preferable. Besides, the cases in which the thickness of the liquid crystal layer is 2.5 .mu.m and 3.5 .mu.m, can be used though not quite satisfactorily.
For example, in the case where the thickness of the liquid crystal layer is made 3 .mu.m, it is required that the roughness of the surface in contact with the liquid crystal layer is made 0.3 .mu.m (300 nm) at most.
If this request is not satisfied, the roughness of the surface in contact with the liquid crystal layer as shown in FIG. 8 has a bad influence on the display.
As one of the methods of solving this problem, it is conceivable that a material such as a resin having fluidity at the film formation is used as an interlayer insulating film 712, so that a difference in level is absorbed.
However, for that purpose, the interlayer insulating film must be made considerably thick.
If the interlayer insulating film is made thick, an opening groove through which the pixel electrode 706 finally comes in contact with the drain electrode, becomes deep. This causes bad contact so that it is not preferable.
Especially, in the case of a miniaturized and minute structure with a small size of 2.5 inches or less in diagonal, such as a liquid crystal panel for a projector, the problem of the contact becomes tangible.
As described above, it is difficult to flatten the surface in contact with the liquid crystal, because of the relation to other requirements.